where \(\Theta _H(x)\)  is the Heaviside theta function, [1]} with \(\Theta _H(0) = 1\) . Accordingly, on the interval \(x \in (y_i, y_{i+1}]\)  we have
\({\Xi ^2(x) }{x} = \sigma ^2_{i+1}[\delta (x_i - x) - \delta (x_{i+1} - x)].\) 
